It is known to detect a spread spectrum chip sequence by correlating a received signal with a reference signal, the peak of the correlation function indicating detection of the sequence. Typically, the detection is performed using samples of the received signal and of the reference signal. Due to sampling, the maximum resolution for determining the position of the sequence in the received signal is ±0.5 sample interval. For example, a sequence transmitted at a chip rate of 2.2 Mchip.s1 and sampled in the receiver at 22 Msample.s−1 would give a detection resolution of ±0.5/22 10−6=±2.27 10−8 seconds. If such a detection process is used for calculating the signal propagation time from the transmitter to the receiver, the accuracy of the propagation time calculation is ±2.27 10−8 seconds. Furthermore, if such a calculated propagation time is used for calculating the distance travelled by the signal from the transmitter to the receiver, the accuracy of the distance calculation is ±c×2.27 10−8, where c is the speed of light, approximately 3.108 m.s−1. Therefore the resulting distance resolution is ±6.81 m.
A higher resolution can be achieved by increasing the sampling rate, but at the expense of increased power consumption and complexity A higher resolution can also be obtained by averaging over measurements made on several occurrences of the chip sequence in the received signal, but at the expense of increased power consumption and increased time delay. In some applications, for example in portable range determining apparatus and portable location determining apparatus for use in an indoor environment, it is desirable to have a rapid high detection resolution and a low power consumption.
An object of the present invention is to provide improvements in the detection of a spread spectrum chip sequence.
According to one aspect of the invention there is provided a method of detecting a spread spectrum signal comprising a chip sequence, comprising sampling at a sampling interval a received signal, filtering in a matched filter the samples thereby obtained, determining the absolute values of the filtered samples, deriving the weighted average values of the absolute values of the filtered samples occurring at intervals equal to the chip sequence length, the weighted average values being calculated over at least two such absolute values, interpolating successive weighted average values thereby generating sub-samples of the weighted average values at a sub-sampling interval shorter than the sampling interval, and determining the position of the chip sequence in the received signal by determining the position of closest match between the sub-samples and samples taken at the sub-sampling interval from a reference correlation function of the chip sequence.
According to another aspect of the invention there is provided a receiver for a spread spectrum signal comprising a chip sequence, comprising sampling means for sampling at a sampling interval a received signal, matched filtering means for filtering the samples thereby obtained, modulus means for determining the absolute values of the filtered samples, averaging means for calculating the weighted average values of the absolute values of the filtered samples occurring at intervals equal to the chip sequence length, the weighted average values being calculated over at least two such absolute values, interpolating means for interpolating successive weighted average values thereby generating sub-samples of the weighted average values at a sub-sampling interval shorter than the sampling interval, and matching means for determining the position of the chip sequence in the received signal by determining the position of closest match between the sub-samples and samples taken at the sub-sampling interval from a reference correlation function of the chip sequence.
By using the interpolated sub-samples at the sub-sampling interval for determining the position of closest match between the chip sequence in the received signal and the reference correlation function, a higher detection resolution may be obtained than using samples at the sampling interval. The higher resolution is obtained without requiring an analogue-to-digital sampling circuit to operated at the sub-sampling rate, thereby avoiding the higher power consumption and increased complexity of such a sampling circuit.
The position of closest match between the sub-samples and the samples of the reference correlation function of the chip sequence may be determined by correlating the sub-samples with the samples of the reference correlation function of the chip sequence.
The time of arrival of the spread spectrum signal may be determined as the determined position of the chip sequence in the received signal relative to a time reference.
The time of arrival of the spread spectrum signal may be determined as the average of more than one determined position of the chip sequence in the received signal relative to a time reference.
The time taken for a radio signal to propagate between the transmitter and receiver may be determined from the time of arrival if the transmitter and receiver have synchronised time references.
The distance between the transmitter and receiver may be determined from the time taken for a radio signal to propagate between the transmitter and receiver.
The interpolation and matching need not be performed over the duration of a complete chip sequence, but may be performed over a shorter duration in the region of a peak in the weighted average values of the absolute values of the filtered samples, thereby avoiding the higher power consumption and circuit complexity of interpolating and matching over the duration of a complete chip sequence.
In one embodiment of the invention the weighted average values of the absolute values of the filtered samples are calculated in accordance with the equation:{circumflex over (χ)}in=α·{circumflex over (χ)}in−1+(1−α)·χinwhere χin is the absolute value of the i th filtered sample in the n th chip sequence,
{circumflex over (χ)}in is the weighted average value of the absolute value of the i th filtered sample in the n th chip sequence,
{circumflex over (χ)}in−1 is the weighted average value of the absolute value of the i th filtered sample in the n−1 th chip sequence, and
α is the averaging gain and has a value in the range 0≦α≦1.
In the drawings the same reference numerals have been used to represent corresponding features.